The present invention relates to an ultrasonic motor driver and, more particularly, to an ultrasonic motor driver using a longitudinal/torsional vibrator assembly as a stator.
A high-frequency voltage as an ultrasonic voltage is generally applied to an ultrasonic motor to drive it. Extensive studies and developments have been made to realize a practical traveling wave ultrasonic motor or a standing wave ultrasonic motor using a traveling wave or a longitudinal/torsional vibrator assembly. Extensive studies and developments of a driving power source have been also made.
By changes in application environment, e.g., an applied external force or temperature, an optimal driving frequency of the ultrasonic motor is changed as a function of time. The change in optimal frequency occurs in all types of traveling and standing wave motors.
A conventional ultrasonic motor driver tracks a change in optimal drive frequency, and has a frequency tracking drive method of changing a practical frequency corresponding to the optimal frequency.
In a standing wave ultrasonic motor using the longitudinal/torsional vibrator assembly, as is well known, when longitudinal and torsional vibrations have a phase difference of 90.degree., the torsional vibration excited by the stator is most effectively transmitted to the rotor. Voltages (to be referred to as excitation voltages hereinafter) applied to the longitudinal and torsional vibrators as ceramic piezoelectric elements are voltages having an appropriate phase difference set such that a phase difference between the vibrations of the vibrators is 90.degree.. In this case, since electric impedances of the piezoelectric elements are different, the phase difference between the both excitation voltages is not always limited to the phase difference of 90.degree.. The phase difference between the excitation voltages, however, is changed as in the driving frequency by the changes in application environment, e.g., the external force or temperature applied to the ultrasonic motor.
FIG. 2 shows an example of relationships among a phase difference .theta..sub.T between a torsional excitation voltage and a torsional excitation current, a current value I.sub.T, and a rotational speed N of the rotor when a phase difference .phi..sub.T between the longitudinal and torsional excitation voltages of the standing wave ultrasonic motor using the longitudinal/torsional vibrator assembly is changed. FIG. 3 shows an example of relationships among a phase difference .theta..sub.L between a longitudinal excitation voltage and a longitudinal excitation current, a current value I.sub.L, and the rotational speed N of the rotor when a phase difference .phi..sub.L between the longitudinal and torsional excitation voltages is changed in the same manner as in FIG. 2.
It is understood that the phase difference .theta..sub.L between the longitudinal excitation voltage and current is not changed at all with the change in the phase difference .phi..sub.L between the longitudinal and torsional excitation voltages as indicated in FIG. 3. As indicated in FIG. 2, the phase difference .phi..sub.T between the torsional excitation voltage and current is changed with the change in the phase difference .phi..sub.T between the longitudinal and torsional excitation voltages. In addition, as indicated in FIG. 2, it is apparent that the phase difference .theta..sub.T between the torsional excitation voltage and current and the rotational speed N are faithfully changed with the change in the phase difference .phi..sub.T between the longitudinal and torsional excitation voltages. That is, when the rotational speed N is maximum, the phase difference .phi..sub.T between the torsional excitation voltage and current is minimized, and when the rotational speed N is minimum, the phase difference .phi..sub.T is maximized.
In the above conventional ultrasonic motor drive method, a driving frequency is changed by tracking the change in optimal driving frequency. There is no countermeasure against the change in optimal phase difference between longitudinal and torsional excitation voltages when the application environment, e.g., an external force or temperature applied to the ultrasonic motor is changed. Therefore, the ultrasonic motor is not always driven in an optimal drive condition because of changes in application environment.